1. Field of the Invention
This invention relates to an apparatus for manufacturing semiconductor devices. More specifically, this invention relates to an apparatus for thermal processing semiconductor wafers within a vertical furnace.
2. Description of the Related Art
In manufacture of semiconductor devices, a plurality of thermal processings are carried out. Examples are thermal oxidation, diffusion chemical vapor deposition and chemical vapor etching. These processings were conventionally carried out by using a horizontal quartz tube. In the horizontal quartz tube, it affords high output of production. In other words, a plurality of semiconductor wafers are processed at once. However, there is a trend to use wafers having larger size. Therefore, the quartz tube has to have large diametric size. This arises one difficulty for maintaining equal temperature distribution in a vertical direction to the horizontal tube. Normally, the upper portion of the tube has slightly higher temperature. Furthermore, large size wafers means increase of total weight of the boat which mounts the such large size wafers. This also arises another difficulty for insertion and drawing back of the boat. Conventionally, a quartz bar is attached to the boat and this bar is handled for the working. The bar tends to bend because of the increased weight of the boat.
Recently, a vertical reactor is also used for saving floor space. In order to install a large number of wafers, the height of the vertical reactor is increased. However, there is a limitation of ceiling of the clean room and this limits the number of wafers which can be set within the vertical reactor.
One example of the vertical reactor is disclosed in U.S. Pat. No. 4,062,318 wherein each wafer is mounted on each surface of carbon susceptors. The vertical reactors are now manufactured by U.S. and foreign makers.
In a U.S. application of Ser. No. 017,693 assigned to the same assignee of the present application, another example of vertical reactor having a plurarity of plasma cleaning electrodes is disclosed.
Other examples of reactors are shown in Japanese Laid-Open patent publications 59(1984)-145531 and 54(1979)-56366. These publications disclose mainly plasma CVD apparatus in which wafers are vertically mounted on a wafer holder within a cylindrical reactor body. Top view of the wafers is spoke-like fashion. Reaction gases are fed at center of the cylindrical reactor. Heat is provided by a lamp provided outside the reactor.